Method for the continuous performance of esterification under thermal conditions

ABSTRACT

A DEVICE FOR CONTINUOUSLY PERFORMING CHEMICAL PROCESSES AND REACTIONS WHICH REQUIRE HEAT ADMINISTRATION FROM THE OUTSIDE AND ARE CONDUCTIVE TO THE EVOLUTION OF VOLATILE BY-PRODUCTS, SUCH AS THE PROCESSES LEADING TO THE PRODUCTION OF SPINNABLE POLYESTERS. THE DEVICE COMPRISES A REACTOR HAVING SUBSTANTIALLY VERTICAL METAL WALLS, MEANS FOR INTRODUCING THE REACTANT AND THE REACTED PRODUCT WHICH IS RECYCLED AT THE REACTOR TOP, CENTRIFUGING MEANS FOR HURLING THE PRODUCTS INTRODUCED AT THE TOP OF SAID WALLS FOR CAUSING THEM TO DESCEND IN THE FORM OF A LIQUID FILM THEREALONG, AND MEANS FOR RECYCLING A FRACTION OF THE PRODUCT WHICH HAS BEEN COLLECTED AT THE BASE OF THE REACTOR, TOWARDS THE TOP THEREOF. A CHEMICAL PROCESS UTILIZING SUCH A REACTOR AND INVOLVING THE REMOVAL OF THE BY-PRODUCTS, SET FREE IN THEIR VOLATILE FORM AT THE TOP OF THE REACTOR AS THEY ARE EVOLVED AND SET FREE FROM SAID LIQUID FILM.

Jan. 22, 1974 slcL ETIAL METHOD FOR THE CONTINUOUS PERFORMANCE OF ESTEHFICATIONS UNDER THERMAL CONDITIONS Filed April 7, 1969 AT roRNEY United States Patent US. Cl. 260-475 P 5 Claims ABSTRACT OF THE DISCLOSURE A device for continuously performing chemical processes and reactions which require heat administration from the outside and are conducive to the evolution of volatile by-products, such as the processes leading to the production of spinnable polyesters. The device comprises a reactor having substantially vertical metal walls, means for introducing the reactant and the reacted product which is recycled at the reactor top, centrifuging means for hurling the products introduced at the top of said walls for causing them to descend in the form of a liquid film therealong, and means for recycling a fraction of the product which has been collected at the base of the reactor, towards the top thereof. A chemical process utilizing such a reactor and involving the removal of the by-products, set free in their volatile form at the top of the reactor as they are evolved and set free from said liquid film.

This invention relates, in general, to the procedures and the means for continuously performing chemical processes, in which, in order that reaction conditions and removal of volatile components and reaction products may be established and/or encouraged an active heat exchange is required and, more particularly, a supply of heat to the reaction mass. The procedure and means considered by the present invention find a particularly advantageous application in esterification, transesterification and possibly also polycondensation processes, as employed for the continuous production of polyesters.

Said field of application, obviously, is but an example of the practical applicability of the invention in the industrial field: the invention can thus be extended to any other field of chemical processes which exhibit equivalent or similar conditions and thermal requirements.

On account of the nature of the present invention, and in order to give a better understanding of its features and objects, the idea of steady conditions, and more particularly of steady thermal condition of the reactions involved will be repeatedly recalled in the following. Attention is directed to the fact that, for the interpretation of such an idea, the criteria and the definitions should be adopted as set forth in the book Pilot Plants, Models and Scaleup Methods in Chemical Engineering-Chapter VI (McGraw-Hill Series in Chemical Engineering) McGraw-Hill, Publishers, New York, 1957.

It is known that, for the performance, more particularly for the continuous performance of processes of the kind referred to, the solution is required of a number of technical problems which often are mutually conflicting, in order that a favorable trading-01f may be achieved between the speed at which the reaction can be carried out (i.e. the quantitative yield of the installation) and the conditions under which the process takes place. The occurrence of secondary or detrimental reactions, liable to alter the qualitative properties of the desired product or anyhow disturb the regular process run should be simultaneously overcome.

3,787,481 Patented Jan. 22, 1974 ICC By way of a short commentary of the foreseen and predominant field of application of the invention, it is known that the chemical processes considered here should comply with well defined requirements. For example, esterification processes by reaction of an alkylene glycol, such as ethylene glycol, with a bicarboxylic aromatic acid, such as terephthalic acid, are accelerated as the temperature is increased. Said increase should be kept, however, within certain limits, to avoid secondary degenerative phenomena, such as pyrolitical phenomena. Moreover, during progress of the main reaction, undesirable second ary reactions such as polyglycol formation could take place to a greater or lesser degree. The lower the proportion of the latter formation, the higher is the speed at which the main reaction is completed, that is, the shorter is the stay time of the reaction mass in the environment in which the process takes place. In addition, the concentration of free glycol has an accelerating action towards polyesterification, and thus said concentration should be kept the lowest possible.

What has been said above holds good also for other cases, for example, in processes of transesterification of diesters of bicarboxylc acids with an alkylene glycol.

In addition, the reaction run is influenced by phenomena of a properly physical nature, which bring about the ease and completeness of the removal of water alcohol or free glycol, in the gaseous state, from the liquid mass wherein the reaction is in progress. Said removal, in fact, is countered by the viscous resistances which oppose the migration of the particles of the volatile component up to the surface of the liquid mass. It can be said that the ease of removal is an inverse function of the thickness of said mass.

Typically, said chemical processes are carried out in a reactor consisting of a metal vessel whose walls are heated from outside, generally by a thermal medium, e.g. biphenyl, which is circulated through a double bottom or jacket which wraps, at least partially, said reactor. As is known, the velocity and the magnitude of heat exchanges between the wall and the reaction mass are proportional to the temperature differential which is established (or maintained) between the inner face of the metal wall (which yields heat) and the liquid layer which adheres to said surface, the latter receives the heat and transfers it to the farther layer of the mass from which the volatile products of the reaction are to be removed.

In view of the foregoing, the principal object of the present invention is to identity, to apply and to use the modes and the means by which both the quantitative and qualitative yield of the reaction can be exalted with respect to the teachings of the prior art, in quite an unpredictable and surprising manner.

For particularly, it is an object of the present invention to provide a method for the rapid performance, as a continuous operation, of chemical processes of the kind referred to above, by establishing conditions under which the reaction goes on under steady thermal conditions which exploit the magnitude of the thermal exchange so as to accelerate the process, and by operating at temperatures which are more favorable to the course of the principal reaction, without however originating detrimental phenomena, such as pyrolysis.

Another object of the invention is to provide a method by which the reaction mass is, and is maintained, in those conditions which are the most favorable ones to the virtually immediate removal of the volatile components.

Still another object of the present invention is to provide an apparatus by which the above enumerated, and other favorable reaction conditions can be established and maintained for the rapid continuous performance of chemical processes of the kind referred to above, and under said favorable conditions.

More particularly, it is an object of the invention to provide a device by which an active heat exchange between the reactor walls and the reaction mass can be ensured, by keeping said mass at temperatures which are close to the maximum temperatures which are consistent with a rapid course of the process without producing any pyrolytic phenomena, said active heat exchange being warranted, on the other hand, by confining to desirably small values the temperature differential between the heatyielding metal wall and the average temperature of the mass adjacent to said Wall surface.

The method according to the invention essentially comprises the metered continuous introduction of the starting and the intermediate materials and compounds into a reactor having heated metal Walls, spraying said materials and compounds against the top of an essentially vertical face of said walls, forming a liquid film descending along said surface, continually removing the reacted product from the reactors bottom, preferably in amounts which are many times in excess with respect to that corresponding to the quantity of product resulting from said metered introduction, the removal, for the subsequent treatments, of said corresponding quantity of the reacted product, the continuous recycling of the excess of said product towards the top portion of said reactor, and the continuous spraying of said excess quantity against said top portion of said surface so as to form a liquid film descending along said surface, said materials and compounds being dispersed within said film and reclining the thermal energy which is continually yielded by said surface.

The foregoing and other more specific features of the invention, along with the showing of the principal advantageous eifects and treatment conditions, will be better understood as the ensuing detailed description proceeds of non-limiting examples of application and use of the device which is also contemplated herein and shown in the accompanying drawing, partly in side view and partly in vertical cross-sectional view, the merely structural parts and ancilliary means and contrivances being omitted, since they can be individually adopted by applying the current knowledge of the art.

As shown in the drawing, the device essentially consists of a reactor formed by a vertically elongate vessel, having an essentially cylindrical outlines and comprising, in general, an upper chamber 10, and a lower chamber 11, having the same diameter as the upper chamber or a lower diameter (as shown) in which a mass 12 is held. The level of said mass determines the stay time in the reactor, expressed in terms of the ratio of the overall amount of material which is present at every instant of time in the reactor, to the amount of reacted product drawn in the unit of time.

The metal walls 13 of the chamber are heated by a thermal medium, for example biphenyl, which, is continuously circulated through a coil 14 having an inlet 15 and an outlet 16. The underlying portion of the reactor can be advantageously heated, in turn, by the same, or another thermal medium circulated through a jacket 17 having an inlet 18 and an outlet 19. The means adapted to assure heating and circulation, at the desired temperatures and rates of flow, of said thermal media, are neither described nor shown herein since they are quite conventional.

At the top of the interior 10 of the reaction chamber, means are provided for spraying onto the upper portion of the inner surface 20 of said chamber the starting compounds and materials and said excess of reacted product. Preferably, said means comprise a rotary assembly 21, supported by a shaft 22 and driven by an appropriate motor 23, said assembly comprising a foraminous container 24 into which said excess of reacted product is continuously fed through a spout 25, and an underlying disc-like body 26 on which the material to be reacted, fed from the outside through a spout 27 receiving its supply through 28 in an amount metered for example by a gear pump 29, is refluxed.

Said container 24, which overlies the rotary disc 26, hurls the liquid product onto said surface 20, thus forming a liquid film 30 thereon, the material tangentially hurled by the edge of the disc 26 being fed into said liquid film.

To the bottom 31 of the reactor is connected an outlet duct 32 which comprises several appropriate removal means such as conveying screws, pumps and the like, not shown in the drawing, said duct being branched at 33 into a branch 37 and an outlet duct 34, controlled by a valve 35, the latter valve being controlled, in turn, by a conventional device 36 which controls the level L of the liquid mass 12 so that the outgoing reacted product U has a rate of flow adapted to keep the said level L constant, consistently with the inlet rate of flow E of the starting materials.

The level L of the liquid mass can also be controlled by conventional devices which act on the incoming material in the tube 28, which have not been shown.

The other branch 37 into which the outlet duct 32 is split, has a valve 38 and leads to a pump 39 by which, through ducts 40 and 41 and a second valve 42, the excess material issuing at 32 is returned to said spout 25 for the formation of said liquid film 30.

Obviously, whenever it is either possible or advisable to promote and/or complete the reaction by a mere falling down of the material introduced from outside only, in the absence of said circulating excess, the above described parts, intended for drawing and circulating said excess, can be dispensed with. These cases can be, for example, polycondensations processes, e.g. of polyesters, polyamide demonomerization treatments or, anyhow, treatments for removing comparatively low-boiling compounds or products and the like.

In addition, the reactor comprises means adapted to maintain the desired temperature for the circulating product. For example, as shown, said means comprise appro priate jacketing of the several ducts through which a thermal medium is caused to flow.

Obviously, the reactor also comprises means, of which a duct 43 associated to a valve 44 is an example, for removing the volatile compounds, such as water and alcohol as produced by the reaction and the gasified glycol. Conduit 43 can be advantageously connected to an ancilliary device for stripping, for example glycol from water or alcohol, for condensing said glycol and recycling it with advantage to the reactor, for example by reintroducing it at 45 into the flow which is being continuously recycled to the rotary foraminous container 24.

A device as described above, or an equivalent thereof, is capable of fulfilling the conflicting requirements outlined above, and of ensuring that the reaction may take place under surprisingly quick and favorable conditions, predominantly under steady thermal conditions.

Bearing in mind, in fact, that the amount of the reacted liquid product which is continuously circulated between the outlet 32 and the spout 25 for feeding the reactor again, is many times a multiple (very often many tens of times a multiple) of the amount discharged at U, along the metal surface 20 of the walls 13, a liquid film 30 is maintained, which forms a mass having a high temperature (due to the heating of the walls 13) in which the starting material is evenly dispersed and by which said material is immediately and evenly heated. Said condition permits, inter alia, to introduce at 27 the starting material at the ambient temperature or, anyhow, at a comparatively low temperature. This is a distinct advantage especially when the starting material is a slurry in which a bicarboxylic aromatic acid is homogeneously mixed with an alkylene glycol.

The subject device permits the best results to be obtained when the reacted material has a low viscosity. In such a case, indeed, the liquid film 30 falling along the surface 20 is extremely thin. This is conducive to two paramount advantages: heat is rapidly transferred through said reduced thickness of the liquid film, the several points of which thus have very close and virtually equal temperatures, and the volatile components, promptly vaporized, are immediately and easily stripped from the liquid mass on account of the extremely short migration paths of the particles which are thus immediately set free from the liquid.

The ready absorption of thermal energy by said liquid film 30 warrants the desired active heat exchange even by keeping at extremely low values the temperature differential between the irradiating metal surface 20 and the liquid which forms said film 30. Said condition thus permits the descending liquid film to be maintained at the temperature most favorable one to the reaction, even though said temperature is close to the one which could produce pyrolytic phenomena (i.e. the maximum temperature consistent with the regular reaction course) without having said surface 20, and thus the limiting layer of the liquid film, which contacts said surface, reach said second dangerous temperature.

Said liquid film 30 can thus be regarded as the principal environment and place of the reaction, the latter taking place under steady thermal conditions. On the other hand, the reaction is completed, due to the purposeful extension of the stay time, within the underlying mass 12, said completion taking place, as a rule, under steady chemical conditions.

It should be observed that, even though a continuous circulation of the predominant fraction of the reacted product exists between the bottom 11 and the top of the reactor, the working conditions of the subject device cannot be compared to those of the so-called recycle processes. As a matter of fact, the product which is deviated through the ducts 37-41 is the same as the product being discharged at U and such a kind of flow can be cornpared, rather, to a continuous and even blending of a product in a homogeneous phase and ensures the necessary mass transfers.

According to another aspect, the liquid film 30 represents the surface of a liquid mass, from which gaseous compounds are set free. Heat exchange phenomena essentially occur within said liquid film 30, so that the liquid mass can be regarded as being present, to the ends of the reaction, in the form of a thin liquid film spread over a heat-yielding surface. To the ends of the stay time of the liquid in the reactor, in correlation with the time during which the liquid mass is subjected to heat exchange, the effective surface of heat exchange corresponds to the product of the geometrical surface of the liquid film 30 by the ratio of the rate of flow of the product circulated through the circuit 37-41 to the rate of flow at the outlet U.

The foregoing discussion can be taken as an explanation of the surprisingly advantageous results of the invention, and, more particularly of the establishment of extremely favorable thermal steady conditions, in which a large amount of thermal energy can be transferred between the heat-yielding surface 20 and the liquid mass held in the reactor, while concurrently keeping within extremely low values, in the order of a few degrees centigrade, for example of less than 6 C., the temperature differential between the temperature of said surface (that is of the molecules which are present in the liquid layer contacting said irradiating surface) and the average temperature of the whole liquid mass. Obviously the ease with which the vaporized fractions are removed from said mass greatly contributes towards a regular and quick reaction course, along with the homogenization resulting from a continuous and repeated circulation.

What is claimed is:

1. A method for the esterification of terephthalic acid with an alkylene glycol which comprises continuously projecting by centrifugal force a liquid mixture of terephthalic acid and an alkylene glycol against the upper portion of the inner surface of a wall of a reactor which has good thermal conductivity, to form a thin film of the liquid on said inner surface that flows down said inner surface,

while contacting the external surface of said wall with a thermal medium to effect the reaction,

continuously removing the reacted product from the bottom of the reactor in liquid state, and

recycling the larger portion of the reacted product to the reactor and projecting it by centrifugal force against the top of said wall above the first named liquid mixture so as to disperse this portion of the reacted product in the mixture of terephthalic acid and alkylene glycol to form therewith the dropping liquid film in contact with the inner surface of said wall which absorbs heat due to contact with said inner surface of said wall,

discharging the lesser portion of the reacted product from the reactor and continuously removing volatile coproducts from said reactor.

2. A method according to claim 1, wherein the excess of reacted product is hurled against said wall at a level higher than that at which the liquid starting mixture is hurled.

3. A method according to claim 1, wherein said thermal medium is circulated at high speed in contact with the external surface of said wall and at higher temperature but close to a predetermined reaction temperature so as to ensure heat exchange during flow of the film down said wall but restrict temperature differential between said inner surface and any point of said film.

4. A method according to claim 3, wherein said temperature differential is less than 6' C.

5. A method according to claim 1, wherein the liquid flowing down said inner wall is collected in the bottom of the reactor, and the level of liquid collected at the bottom of the reactor is controlled so as to establish the stay time in the reactor.

References Cited UNITED STATES PATENTS 3,644,294 2/ 1972 Siclari et a1 260-475 F FOREIGN PATENTS 23,564 10/1964 Japan 260475 P LORRAINE A. WEINBERGER, Primary Examiner E. J. SKELLY, Assistant Examiner US. Cl. X.R. 

